It is possible to arbitrarily set the amplification degree for the sound signals from the microphone of a conventional portable telephone set before the portable telephone set is delivered as a final product. Once this amplification degree is preset, however, the telephone set can be used only at the preset degree. The telephone set may move with the user to various places, and be used in various environments. Because the amplification degree is preset, however, the telephone set is not easily adaptable to various aural environments. If the preset amplification degree is relatively low, the user may speak louder naturally in a noisy environment, bothering or annoying other people.
In connection with such a problem, Japanese Patent Laid-Open No. H.6-6441 discloses a portable telephone set with which the user can speak lower in a public environment in order not to annoy the public. The portable telephone set is provided with a manual switch for setting a public environment mode, when the user uses it in a public environment. When the public environment is set by turning the manual switch on, transmitting and receiving sound amplifiers are increased in amplification degree, and a sidetone amplifier is turned on to provide the user with an amplified sidetone.
As a result, in the public environment mode, the sidetone is louder when the user is speaking. This naturally makes the user speak or vocalize lower in this mode. In this mode, the amplification degree of the other sound signal amplifier 9 is higher. Therefore, in this mode, the volume of the sound heard by the other party is not affected even if the user speaks low.
In the public environment mode, unconditionally, the amplification degree of the sound signal amplifier 9 connected to the microphone 11 is increased, and sidetone is output to be increased in level from the earphone 5. This prompts the user to speak lower. Therefore, in a very noisy environment, the user speaks low and the amplification degree of the sound signal amplifier 9 is higher, and consequently the other party hears more noise.
In a digital portable telephone set or the like, high-or dense-compression sound coding technique is used. In this case, for simple coding of non-voice, when the noise level is high, the collected sound is coded as a sound signal of small volume. Consequently, if the user speaks low in a noisy place, the other party may hear a deteriorated voice. As a result, the party may ask the user to speak loud, or to cancel the public environment mode.